Sheet metal is typically formed by hot rolling process and, if the gauge is thin enough, it is coiled for convenient transport and storage.
The most common method of removing all oxide from the surface of hot rolled sheet metal is a process known as pickling, or pickle and oil. In this process, the steel (already cooled to ambient temperature) is uncoiled and pulled through a bath of acid to chemically remove the scale. Then, after the scale has been removed, the steel is washed, dried, oiled to protect it from rust damage, and recoiled.
During the pickling process, the sheet metal passes over and/or through a plurality of rollers and may encounter one or more gaps as it travels through the process line. In other words, the surface over which the sheet metal passes during the pickling process is not solid, but may contain recesses such as gaps, crevices, or edges. In the past, problems have occurred when the leading edge of the sheet dips into a recess and forward progress is impeded. Having to manually reach into the process line in order to free the leading edge of the sheet metal can be a safety hazard for line operators.